I heard that a bad karma erasing mantra associated with Ksitigarbha (kind of like how om mani padme hom is associated with Avalokiteśvara) is "om ha ha ha vishmaya svaha". Does anyone know anything about this? If so, would you happen to know the translation for this mantra in Pali?

You can try that question at the sister site here where it is dedicated to anything and everything that is Mahayana/Vajrayana. This forum is dedicated for the Theravada practice and teaching and as such translating mantras into the Pali language may not serve that purpose.

Bhikkhus, if you develop and make much this one thing, it invariably leads to weariness, cessation, appeasement, realization and extinction. What is it? It is recollecting the Enlightened One. If this single thing is recollected and made much, it invariably leads to weariness, cessation, appeasement, realization and extinction.Anguttara-Nikaya: Ekanipata: Ekadhammapali: PañhamavaggaVSMVMMWBBTBHTWTBTMy Page

I don't know of any Pali chant or formula for this bodhisattva. He appears to be of significance mostly in Mahayana. But I did find this at his wikipedia page:

In Theravada Buddhism, the story of a Buddhist monk called Phra Malai, who has the similar qualities of Ksitigarbha, is well known throughout Southeast Asia, especially in Thailand and Laos. Legend has it that he was an arhat from Sri Lanka, who achieved great supernormal powers through his own merits and meditation. He is also honored as a successor to Maudgalyayana, the Buddha's disciple foremost for his supernormal attainments. In the story this pious and compassionate monk descends to Hell to give teachings & comfort the suffering hell-beings there. He also learns how the hell-beings are punished according to their sins in the different hells.http://en.wikipedia.org/wiki/Ksitigarbha